1. Field of the Invention
The present invention relates to a heat fixing device for use in image forming apparatuses, such as copying machines, printers and the like, for fixing a toner image to a receiving medium. The invention is characterized by an increased length of a nip portion for heat fixing the toner image to the receiving medium and by an arrangement directed to an efficient use of heat.
2. Description of the Related Art
Conventionally, the image forming apparatuses, such as copying machines and printers, operate to transfer the toner image onto the receiving medium and to fix the toner image to the receiving medium by means of a fixing device.
Heretofore, widely used fixing devices have been arranged as shown in FIG. 1. That is, a receiving medium 1 supplied with a toner t is introduced to place between a pair of fixing rollers 2, 3 which cooperate with each other to fix the toner t to the receiving medium 1. The above fixing device includes heating elements 4, 5 disposed in the respective fixing rollers 2, 3 for ensuring that the toner t is fully fixed to the receiving medium 1. Furthermore, the fixing device employs the fixing rollers 2, 3 constructed such that a core 2a, 3a formed of a metal is formed with an elastic layer 2b, 3b on an outer periphery thereof, the elastic layer formed from an elastic material such as rubber. As heated by the heating elements 4, 5, the fixing rollers 2,3 are pressed against each other so as to define the nip portion of an increased length between the elastic layers 2b, 3b overlaid on the outer peripheries of the fixing rollers 2, 3. Thus is ensured that the toner t is subjected to sufficient heat and pressure to fix the toner t to the receiving medium 1.
More recently, a demand exists for further speed-up of the above image forming apparatuses. This dictates the need for quick and positive fixing of the toner t to the receiving medium 1. In a full-color image forming apparatus, on the other hand, toners t of multiple colors are supplied to the receiving medium 1 to form thereon a thick layer of the toners t, which need be fully fixed to the receiving medium 1.
In order to permit the fixing device to fix the toner t to the receiving medium 1 in a quick and positive manner or to fully fix the thick layer of the toners t to the receiving medium 1, the fixing rollers 2, 3 need to define therebetween an even longer nip portion where the toner t is heated and pressurized.
As an approach to increase the length of the nip portion defined between the fixing rollers 2, 3 it may be contemplated to increase the diameter of the fixing rollers 2, 3 or to increase the thickness of the elastic layers 2b, 3b overlaid on the outer peripheries of the fixing rollers 2, 3.
Unfortunately, in a case where the fixing rollers 2, 3 are increased in the diameter, as described above, the fixing device is also increased in size or the receiving medium 1 is more liable to wind about the fixing roller 2 contacting the toner t on the receiving medium. In a case where the elastic layers 2b, 3b formed on the outer peripheries of the fixing rollers 2, 3 are increased in the thickness, on the other hand, heat transfer to outside surfaces of the fixing rollers 2, 3 is lowered so that it takes more time and a substantial quantity of electric power to sufficiently raise the temperature of the outside surfaces of the fixing rollers 2, 3. This results in an increased running cost.
More recently, there has been proposed a heat fixing device arranged as follows. A pair of fixing belts drivably looped about a respective pair of rollers are heated by means of heating elements, while a receiving medium supplied with a toner is introduced into a nip portion defined between a pair of fixing belt portions brought into tight contact by the roller pairs, the nip portion serving to heat and pressurize the toner for fixing the toner to the receiving medium (see, for example, JP-A No. 9-274401).
In such a heat fixing device, however, the fixing belts are driven as looped about outer peripheries of the respective pair of rollers. Therefore, the fixing belt has an increased contact area with the roller pair so that the heat of the fixing belt applied by the heating element is detrimentally absorbed by the rollers. Consequently, the heating element is decreased in the efficiency of heating the fixing belt, requiring a substantial quantity of electric power for sufficiently heating the fixing belt.